(1) Field of the Invention
The present invention relates to a method of designing an optical scanner used in an image forming apparatus such as a laser printer, and more particularly, to a method of designing a post-objective type optical scanner wherein a converged light beam from a condensing lens is directed to an optical deflector which effects scanning thereof, and to a product made by the method.
(2) Description of the Related Art
An optical scanner widely used in conventional laser printers is a pre-objective type in which a condensing lens is placed in an optical path between a polygonal mirror and a photosensitive drum. The polygonal mirror is one of the optical deflectors which deflect a beam of light directed thereto and effect scanning thereof on the photosensitive drum as they rotate around the axis of rotation. The condensing lens is a lens through which the incident light beam is converged to a point on a scanning surface(the surface of the photosensitive drum) in a scanning direction, and in general, it is composed of a series of lenses. Unlike the other general purpose lenses, the condensing lens used in the optical scanner must hold a specific characteristic.
It must hold a characteristic known as the f.multidot..theta. characteristic: an angle of scanning is proportional to a scanning width. In addition, it must be capable of compensating for curvature of field both in the scanning and sub-scanning directions as well as the f.multidot..theta. characteristic. The curvature of field is a shift of a point of convergence from on a plane to on a curved plane, which results in a curved image. In FIG. 1, the curvature of field in the scanning direction or in the sub-scanning direction with the conventional pre-objective type optical scanner is illustrated.
As is previously explained, the condensing lens composed of a series of lenses is placed in an optical path in such a way that the deflected light beam is incident thereon. Hence, it is relatively easy to design the condensing lens capable of compensating for the curvature of field in both directions and f.multidot..theta. characteristic. On the other hand, however, such compensation can not help enlarging the condensing lens, and thus aligning the condensing lens with a series of lenses becomes intricate, which leads to the increase of manufacturing costs. Moreover, such compensation requires a relatively long optical path, making it almost impossible to assemble a smaller optical scanner.
Given these circumstances, actively researched and commercialized in recent years as a compact and inexpensive optical scanner is a post-objective type in which the condensing lens is placed in an optical path prior to the optical deflector. In the optical scanner of this type, as is in the pre-objective type, a capability of compensating for the curvature of field in the both directions as well as the f.multidot..theta. characteristic is essential.
Japanese Laid-Open Patent Application No. 1-169422 discloses one of the conventional post-objective type optical scanners, of which schematic view is in FIG. 2.
Numeral 22 is a condensing lens, and a beam of incident light from a semiconductor laser 21 is converged to a point on the scanning surface of a photosensitive drum 26 in the scanning direction. Numeral 23 is a cylindrical lens with a power in the sub-scanning direction, so that the incident light beam from the condensing lens 22 is converged to a point in the vicinity of a reflecting surface of a polygonal mirror 24 in the same direction. The polygonal mirror 24 is composed of cylindrical surfaces, and it effects scanning of the light beam from the cylindrical lens 23 directed thereto on the photosensitive drum 26 by deflecting the light beam as it rotates around an axis of rotation 27. Numeral 25 is a compensating lens with a power in the sub-scanning direction, so that the incident light beam from the polygonal mirror 24 is converged to a point on the scanning surface of the photosensitive drum 26 in the same direction. The photosensitive drum 26 is a photosensitive body exposed when subjected to radiation of the light beam, and numeral 27 is the axis of rotation of the polygonal mirror 24.
The operation of the conventional post-objective type optical scanner constructed as above is explained with referring to FIGS. 3(a) and (b), wherein optical paths in planes in the scanning and sub-scanning directions are illustrated, respectively.
A beam of light from the semiconductor laser 21 incident on the condensing lens 22 is converged to a point on the photosensitive drum 26 in the scanning direction, which is subsequently directed to the cylindrical lens 23 and converged to a point in the vicinity of the reflecting surface of the polygonal mirror 24 in the sub-scanning direction. The incident light beam from the cylindrical lens 23 is directed to the polygonal mirror 24 which effects scanning thereof on the photosensitive drum 26 by deflecting the light beam as it rotates around the axis of rotation 27; the reflecting surfaces thereof are cylindrical, so that the compensation for the curvature of field in the scanning direction is completed upon the deflection. The incident light beam on the compensating lens 25 from the polygonal mirror 24 is converged to a point on the photosensitive drum 26 again in the sub-scanning direction.
In summary, the compensation for the curvature of field in the scanning direction is made possible by designing the surface of the polygonal mirror 24 to be a cylindrical or spherical surface instead of straight surface, while that for the curvature of field in the sub-scanning direction and f.multidot..theta. characteristic is made possible by placing the compensating lens 25 in an optical path between the polygonal mirror and scanning surface.
In addition, the compensating lens 25 is used to compensate for a tilt of the reflecting surface of the polygonal mirror 24 by designing a point of deflection to be geometrically conjugate with the scanning surface. The compensation for the curvature of field in the sub-scanning direction and f.multidot..theta. characteristic is almost perfect with the compensating lens 25. However, the compensation for the curvature field in the scanning direction is inadequate with the polygonal mirror 24. As a matter of fact, the curvature of field in the scanning direction is acknowledged up to 2.7 mm. It may be possible to provide a new component to improve the compensation for the curvature of field in that direction, but accompanying with this, adverse effects are likely to be brought about on the almost perfect compensation of the curvature in the sub-scanning direction and f.multidot..theta. characteristic. Therefore, a method of designing a post-objective type optical scanner capable of compensating the curvature of field in the scanning direction without adding any new component has been sought after.